1. Field of the Invention
The present invention relates to a method for operating a vehicle brake system, and more specifically to a method for limiting brake pressure to a predetermined pressure value.
2. Description of Related Art
Numerous embodiments of vehicle brake systems exist. Anti-lock brake systems (ABS systems) are often part of an electronic stability control system (ESP). Brake systems may vary from the classic hydraulic dual circuit brake systems with a tandem master brake cylinder and hydraulic brake booster to electric motor supported brake systems or Brake-by-Wire systems (BBW). Brake-by-Wire systems using limited mechanical or hydraulic connections between the brake pedal and the brakes. In principle, Brake-by-Wire systems allow greater influence on the brake system by the vehicle control systems.
In a heavy braking process, in particular emergency braking, a conventional brake system rapidly builds up brake pressure, wherein the brake pressure often rises to high values within the specifications of the brake system.
Often the braking effect resulting from the high brake pressure is not fully converted into a corresponding deceleration of the vehicle, because the deceleration depends in particular on the respective road and weather conditions and the resulting adhesion coefficient μ. If the brake force can no longer be converted, wheel slip or wheel locking occurs, which is counteracted in a known way by the ABS, including a targeted reduction of the brake forces on individual wheels or a group of wheels. During a braking process, a situation can develop dynamically. For example when changing from a road surface with high μ (with correspondingly good traction conditions) to a road surface with lower μ (poorer traction conditions—such as wet, snow or ice etc.), often referred to as a negative μ-step. Vehicle dynamics control must react to this rapidly by pumping out low pressure brake fluid accumulators to counteract locking of the rear wheels. Reducing brake pressure, to avoid locking of the individual wheels, can be a problem at high brake pressures.
Further, especially at lower μ values, high brake pressures may cause an unwanted increase in the NVH values (NVH=Noise, Vibration, Harshness).